1. Field of the Invention
The present disclosure relates to techniques for acquiring downhole seismic data of a formation, and, more particularly, to techniques using a multi-level seismic tool to acquire seismic data simultaneously at multiple location or depths.
2. Background of the Related Art
Existing Borehole Seismic While Drilling tools contain seismic receivers with a highly accurate downhole clock. The seismic sensors are disposed in a drill collar to record borehole seismic data while drilling ceases, as shown in FIG. 1. The tool digitizes the received seismic signals or waves and puts the data into a circular buffer. In other words, as the tool is drilling seismic data is typically buffered in an on-going basis keeping only the most recent acquisitions. As soon as the tool recognizes the first break/indication of a desired seismic signal or cessation of drilling, the tool acquires data from a specified time before the first break or cessation in a predefined time window. Thus, the tool is able to record a seismic signal that was received during a period of time that is deemed to be the time window in which the relevant data should appear. The tool then stores the data with a time stamp based on the high precision clock. The tool then transmits the first break time, and possibly some data (possibly after data compression), to the surface by using a MWD telemetry system. Using the downhole first break time and the surface reference time, the travel time for seismic energy between the surface and downhole may then be computed.
Since seismic data cannot be acquired while drilling because of high noise, acquisition is performed each time drilling ceases. For example, when adding a new stand, which is typically three joints of a drill pipe. The typical length of a drill pipe joint is about 10 m and the length of a stand is therefore typically 30 m. This means that the seismic data is acquired at every 30 m. In contrast, conventional wireline borehole seismic measurements are acquired at 15 m intervals to optimize spatial aliasing in Vertical Seismic Profiling data. Therefore, in order to obtain the benefits of the 15 m intervals using conventional single level or single module while drilling seismic tool, the drilling has to be stopped in the middle of running the stand, just to take the measurement. Such an operation is not preferred because of rig time is expensive and additional downtime or non-drilling time is costly.
A further limitation of the current Borehole Seismic While Drilling tools is the clock drift that occurs when the tool has been drilling for an extended period of time. In other words, once a tool has been drilling for several days (three or more for example), the downhole clock becomes desynchronized from the uphole or reference clock, the difference in the synchronization being the drift. This drift then causes inaccuracies in the interpretation of the received data. Currently, in order to compensate or eliminate the drift, the tool is brought back to where a previous checkshot was completed when the clocks were still synchronized, so that the clocks can be recalibrated or resynchronized. Since the drill pipe has to be pulled up, or possibly some joints of the drill pipe have to be removed at the surface to lift the tool to the depth where the previous checkshot was performed, the clock calibration requires rig downtime which, again, is expensive.